1. Field of the Invention
This invention relates to ignition devices of the form which are known, in the context of aero gas turbine engines at least, as torch igniters. The torch igniter which constitutes the present invention is particularly intended for use in a gas turbine engine such as an aero engine or a marine engine but it could possibly find use in other additional applications such as boilers or furnaces.
2. Discussion of Prior Art
Gas turbine engine igniters are of two separate types. Both utilize what is termed herein a "flame lighter" that is a device such as a spark or surface discharge unit for supplying the heat to initiate combustion. In the first type of igniter the flame lighter is situated within the combustion chamber such that it is within the influence of the main fuel spray. The second type of igniter is the torch igniter with which this invention is concerned and this igniter form comprises a flame lighter co-located with a fuel atomizer or sprayer which is auxillary to the main sprayer. This type of igniter is well established in the art being utilized particularly in vapourizing combustors which rely on the heat of combustion to evaporate the fuel of the main supply thus necessitating an independent source of suitably atomised fuel for ignition purposes.
Torch igniters are less sensitive to location within the combustion chamber than are igniters reliant on fuel from the main sprayer and accordingly they can often be positioned such that they are not exposed to the most extreme conditions present within the combustion chamber. Such siting of the torch igniter may avoid the worst of the damage phenomena to which the other igniters are subjected. However the conventional torch igniter which utilizes a fuel injector, such as a swirl atomizer or fan sprayer, mounted alongside the flame lighter is very susceptable to blockage of the injector because it has a fine exit hole or passage at the tip adjacent the flame which is easily obstructed and because at high engine power the fuel within the atomizer is exposed to temperatures at which the fuel can `crack` to produce gums or coke. It is conventional to maintain the fuel flow through the torch igniter fuel injector in order to cool the injector and thus minimise risk of blockage but this produces secondary problems. The burning fuel from the torch igniter, during normal running of the engine, may generate `hot streaks` in the gases within the combustion chamber which are close to the combustion chamber walls and the flame lighter and which cause problems for wall cooling and lighter life. Soot or coke from the burning fuel may degrade the operation of the flame lighter by formation of deposits on or near the flame lighter surface.
The torch igniter is only required to function during the start-up phase of engine operation and in any altitude relight which might be required. The fuel supply to the torch igniter could therefore be shut off during normal running of the engine with benefits to the efficiency of the engine and the life expectancy of parts of it providing the problems of blockage in the torch fuel injector could be solved.
It has previously been proposed to do this by utilizing purge air in connection with a conventional torch igniter. This is not attractive at least in aero engines because a separate supply of `bottled` purge air is required in order to supply the necessary purging pressure, and the additional equipment weight and operational complexity outweigh the benefit achieved.